linux-kernel - Re: [PATCH 0/4] [GIT PULL] tracing: recursion and compile fixes

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Date:	Mon, 20 Apr 2009 23:01:53 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 0/4] [GIT PULL] tracing: recursion and compile fixes

On Mon, Apr 20, 2009 at 04:44:46PM -0400, Steven Rostedt wrote:
> 
> On Mon, 20 Apr 2009, Frederic Weisbecker wrote:
> > > > 
> > > 
> > > Hmm, doesn't the trace wakeup test if the runqueue lock is locked or not?
> > > 
> > > -- Steve
> > > 
> > 
> > 
> > Hmm, yes it does but that's not the first time we meet this problem
> > (sched switch event tracing recursions by the past). So either the
> > test doesn't work well or this is about another lock that
> > wake_up_common takes...
> 
> Ug, it is the task's rq lock. Not the current rq lock. wakeup takes the 
> runqueue lock of the task. The "runqueue_is_locked" only tests the lock of 
> current CPU, which is not what we can have.


You mean the lock held on the wait_queue for wake_up_trace() ?

void __wake_up(wait_queue_head_t *q, unsigned int mode,
			int nr_exclusive, void *key)
{
	unsigned long flags;

	spin_lock_irqsave(&q->lock, flags);
	__wake_up_common(q, mode, nr_exclusive, 0, key);
	spin_unlock_irqrestore(&q->lock, flags);
}


> 
> Thus, the function tracer (gag, and probably the event tracing!) should 
> not call wakeups.


No problem for the events, I made them using the nowake commit because
of sched switch recursions :-)
That's why the nop tracer is now a "polling on traces" tracer.


> -- Steve
> 

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